Constantine had hoped that he would find it uniting to have Christianity as the State religion, but he had not reckoned with the disagreements between the various factional Churches which often claimed jurisdiction over the same areas, and threw the accusation of heresy at their fellow Christians, often leading to vicious street fighting between adherents of the various factions. Of course there had always been disagreements between Christians. St Paul makes that clear in his letters, but the Christians had never before had the power of the state behind them, and from which they expected support.
Constantine was displeased, and the displeasure of a despotic emperor is to be avoided. So he called a meeting of the leaders of the Church(es) at Nicea in 325CE and by his personal attendance ensured peace, harmony, and acceptance of his wishes and decisions about what Christianity should teach even though he knew no theology at all!
However anyone who disagreed with him was pronounced anathema or heretical and banished. This situation rapidly brought unanimity among the Christians in the Roman Empire, although the defeated and banished Christians went on, in the main eastwards, to establish a great and thriving Church which held their own beliefs.
Because of the power and centralization of the Roman empire we in the west have only fairly recently come to realize that there were, and are, Christians other than those of which we have been generally aware, although via trade links they have occasionally popped into our histories, like the priestly envoy from China’s emperor Hulagu to the Vatican, who came to England and from whom King Edward 1 received Communion, and a Nestorian bishop ordained in 1889 a Russian who eventually became an Orthodox Bishop.
For the rest of his life Constantine led and guided the Church in the West so that when he died it was a fixed state body, which had pretty well a uniformity of belief and authority, and although there was to be much debate yet on what was true Christian belief, the bones were established. They were fleshed out by the great Ecumenical Councils, which finally determined what had to be believed for a person to be saved.
Notice that now membership of the Church was becoming a matter of belief rather than practice as it had been earlier.